Repeater circuits



v Jan. 5 1925'. 1,568,891

L. EsPENscHlED v REPEATER CIRCUITS 'Filed vJuly 2. 1921 2 sheetssheets mmto@ Patented JJan. 5, 1926.

UNITED ASTATES i 71,568,891 PATENT' OFFICE. A

LLOYD ESPENSCHIED, F QUEENS, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

REPEATER CIRCUITS.

Application filed July 2,

tain Improvements in Repeater Circuits, of

which the following is a specification.

This invention relates to transmission circuits, and more particularly to transmission l circuits' of the general type exemplified by two-way repeater circuits.

1 line sections, the well-known hybrid coil Iii-repeater circuits of the so-called 22-v type, it has been customary to differentiate between the sending and receiving channels bybalancing the channels with respect to the and artificial line or networkl being employed for this purpose. ,The problem -of balancing a line' involves grave difficulties, however, and frequently the degree of bal- 0 ance is so poor that the amplification must receiving channelson a time basis,-that is,

3,- in switching the line at both ends of the re be maintained quite low to preventsinging.

In accordance with the present invention,-

it is proposed to effect two-way transmission by differentiating between the sending and yby associating the channels with the line alternately at a vfrequency high 'as' compared with the signaling frequency. This results peater at a rapid rate between the sending and receiving terminals of the repeater. This 'will obviously permit of. operating the re- `peaterat very 'high gain without encountering difficulties diie to`singing, since the probllem ofA repeater singing is essentially oneof discriminating, between the oppositely .directed channels, This method of switching may be used either to supplement the discrimination due to' balance or may be substitutedffor'the balancing method.

An important feature of the present invention resides in the employment of 'vac- 4uuin tubes for performing the switching function, the vacuum tubes havingv in this connection the characteristic advantage, as compared with mechanical switching means, of substantial absence of inertia, thereby permitting of high speed switching and absence of contacts with their attendant troubles. j

More specifically considered, the invention involves an arrangement whereby the vacuum tube amplifiers in the oppositely directed channels of the 22 repeater circuit are lalternately rendered operative at a high fre- 1921. serial No. 482,243.

quency rate by impressing proper operating potentials upon the grids of the tubes. These potentials may be applied by so connecting the circuits of the two tubes that singing will be set upand the alternating current pulses ofthe singin oscillations will alternately render the tu es capable of amplification. As an alternative, the alternating impulses for rendering the tubes operative may be applied from some external source. In either case, the circuits connecting the two channels shouldbe arranged to discriminate against currents of signaling frequencies so as to prevent singing at a signaling frequency. n In order that the commutating action of the pulses applied to the grids of the amplifiers may be made to involve as short a time as possible for the switch-over operation, it

is proposed to distort the wave which is applied to the amplifier so that it becomes a flat topped wave. tube amplifying during the period that one flat topped alternation is being transmitted,

While the tube will be rendered non-amplifying by the flat topped alternation of opposite polarity.

Further details of theinventionrwill be.

clearly understood from the following description, when read in connection with the accompanying drawing, Figures 1 and 3 of which illustrate two forms of the invention This will permit of thev in which the amplifiers themselves are caused l `to oscillate to supply the commutating currents, and Figs. 2 and 4 of which are curves illustrating the operation of the circuits of Figs. 1 and 3, while Fig. 5 is a modified arrangement in which the commutating cur- Referring to Fig. 1, th'e usual form of 22 Y repeater circ-uit 'is shown, in which line sections L and L are associated with repeater channels RE and RW through hybrid coils 10 and 10', artificial lines or networks N and N being provided to balance the line sections. l Amplifiers AE and AW are included in the repeater channels RE'and RW, respectively.. While these amplifiers may be of any well-known type, they are preferably of the vacuum tube type. In order that both amplifiers ma not function to amplify with efficiency at t e same time, but may 'be reiidered operative as amplifiers alternately, the

vnon-linear portlon of the characteristic curve. The amplification of the tubes at telephone frequencies will, therefore, vary in accordance with the amplitude and polarity of the high frequency waves which oscillate in the tuned circuits F and F due to the singing connection established between the two tuned circuits and the amplifying tubes. The amplifications of the amplifiers AE and AW are consequently varied alternately by the high frequency singin first one and then the other of the ampli ers coming into play to amplify the telephone currents. By means of this arrangement, the total amplification around the repeater circuit, (that is, around the path in whichl low frequency singing tends to occur as the result of line impedance unbalances), is kept at a relatively low value so that audio frequency singing is minimized. The action of the repeater may be described as one in which the repeater gives continuously a small gain around the telephone repeater circuit while intermittently, but in very rapid succession, relatively lar e gains are alternately given v to thevchanne s east and west.

In order that the operation above described may be obtained, the grid batteries of the amplifying tubes may be adjusted in either of two ways depending upon their phase relation in the oscillating circuit. If, for example, the selective circuits F and F are of such a character as to give little or no phase difference between the amplifiers AE and AW, then `one amplifier may be operated on one knee of the characteristic curve, as for example` onthe point a of the characteristie curve of Fig. 2, and the other amplifier may be operated upon the opposite knee of the curve, as, for example, at the point b in Fig. 2. For this condltion, a half-wave of the high frequency passing substantially simultaneously through the two tubes will diminish the amplification of one tube and in `crease the amplification of the other, and

vice versa for the opposite half of the high frequency wave.

If, on the other hand, the feed-back selective circuits F and F are connected in the reverse manner so that a positive half-wave in the tube AE Acorresponds to a negative half-wave in the tube AW, then both tubes should be operated on the same knee of the characteristlc curve, as, for example, at the point a. One amplifier will their have its gain as regards teleplionic transmission increased, while that of the other Will be decreased.

Instead of using sharply .tuned circuits for obtaining the slnging action described in connection with Fig. l, band filters of the Campbell type may be employed as illustrated in Fig. 3, these filters having the advantage that they may be utilized in controlling the phase relations. The band filters F and F in Fi 3 should be so designed as to transmit a s arply defined p edominant natural frequency above the te ephone range. This may be accomplished by making the band of free transmission of the filters quite narrow. If the filters are further so designed as to have an electrical length of one-quarter wave length, a phase difference of 180 degrees will exist between lthe tubes AE and AW. In this case both tubes should be operated on the saine knee of the characteristic curve. For example, as illustrated in the characteristic curve of Fig. 4, the grid batteries C and C should be adjusted to apply a potential Ec to the grid of each tube, thereby causing the tubes to operate at the point a on the characteristic curve. If now a positive half-wave of amplitude@ is superposed upon the normal potential Ec, the tube will operate at the point f of the characteristic curve, at which point the tube is substantially non-amplify- I mg. If, on the other hand, a negative halfwave of amplitude e is superposed upon the normal grid potential, the tube will be caused to operate about the point g, which is the point of greatest amplification of the tube. Since these two actions in the tubes AE and AW occur at phase intervals of 180 degrees, the tubes will alternate in their amplifying action. It will be understood, of course, that this phase difference may be brought about by making thev filters F and F of other electrical lengths than'one-quarter wave length each, as above described.

In the arran and 3, the high requency control current for the amplifiers was generated by the amplifier tubesthemselves. The general scheme involved would probably be more. practical for telephone use if the hi h frequency control current were generated separately from the amplifiers themselves by means, for example, of a high frequency oscillator. This would eliminate the complication -of causing the repeaters to function simultaneously as oscillators while functioning alternately as amplifiers. The provision of a separate high frequency sourcewould, furthermore, not involve any -considerable additional expense, as the same source might be used for a plurality ofrepeaters. f Fig. 5 illustrates un amplification control systemin which the controlling high frequency current is izo ,- t emsel'ves.

enerated se arately from the re aters his figure introduces a urther feature of importance in that "it provides means for minimizing the distortion of the telephone currents which would normally-be occasioned by the action of runnin the repeater up and downits non-linear c aracteristi'c. v

Referring to Fig. 5, two amplifiers AEl and AW are supplied from acommon vacuum tube oscillator O, which applies cur- ,rents to the tubes in opposite phase relation.. The general action is the same'as -in Figs. l and 3, the amplifier AE being caused to ,operate upon( theiportion of its characteristic curve giving minimum amplification, while Iat the same time the amplifier AW is caused r to operate' upon the portion of its characteristic curve corresponding to maximum amplification.

If the current supplied to the amplifiers from the source O were of sine wave form, the amplifiers would be continually shifted in positionv on their characteristic curves. As a consequence, during the greater part of the time the' repeaters Vwould be operating on portions of their characteristic curves I other than the straightest or most linear Apart, -so that the voice'currents traversing 'the tubes would be subjectedto distortion. In accordance with the present" invention,

' this, difiiculty due to distortionmay. be over- 'come by supplying to the amplifiers waves y 'having square .tops instead of sinusoidal t waves. Byusing a square topped wave, the

amplifier maybe shifted from maximum to minimum amplification inthe shortest time possible so. as to minimize the distortion of the voice 'currents which arises during they-f* time that the repeater is being Vshiftedon its characteristic curve. y

In order to accomplish this result, the Waves supplied by the y oscillator O. .are

passed .through a distorting tube DT having [a 4resistance 20 in its output circuit. The

tube DT and its associated circuits are arranged in a well-known manner so that'the tube will have a characteristic curve such as is indicated in Fig. '(,the change'from the fiat part of1 the, 'curve at saturationl and at the no-current 'L points being such as is indicated in Fig. 6 Vis generated bythe oscillator O, such current, in passing throughithe tube DT, will be distorted so'v that the waves will have Hat tops, as indicated by the curve of Fig. 8.

The` circuits including thev lfilters F and F', which, associate the output circuit ofthe distorting tube DT with the grid circuits of the amplifying tubes AE and AW, respectively, are conductively associated with the output circuit of the distorting tube by -beinof bridged across a resistance 21, Aso that the ldirect current pulses of the waves of relatively abrupt. lf, therefore, a sinusoidal current wave, etc.-

FigfS' will be permitted vto flow into these circuits without -rounding ofi the tops of the 4waves. For tlve same purpose the con-k nections to the grid circuits of the amplifiers 1 AE and AW are established through resistances 22 and 22', respectively; The high pass filters F' andF should be `designed so that their lower cut-ofi point will be below the fundamental `frequency of the square topped Wave of Fig. 8, but the lcut-olf should be sufficiently high so as to exclude audible frequencies.` \-The .Suppression of audible frequencies by these `filters will eliminate: any possibility of the' establishment of a singing connection at audible frequencies for either amplifying tube ove'i` the output circuit of the tube through a hybrid I coil and through the filters F and F to the input circuit. By' making the lower cut-off point low enough to include the fundamental frequency of the Wave of Fig.

bev transmitted t rough the filters without any roundingof the tops of the waves'owing to'tlie fact that the square topped form of wave may be considered to comprise a fundamental frequency upon which are superposed successive harmonics.' As the high pass filter will pass the fundamental wave and all of its harmonics, the s uare to ped form of the Wave'will not be disturbed.

8, the square top ed form of the wave may j Ifthe grid batteries of the amplifiers are adjusted to operate normally -at :the point m, which corresponds tothe lower knee of the characteristic curve of Fig. 9, ,the-su rpositionk of the alternating square toppe wave of Fig. 8, upon the grid will result in 'shiftingthe' operating point along cycleand to the point p during the other half-cycle. At the point. n, the tube will be operating at its greatest amplifying .efficiency and at the point vp will be operating at substantially noy amplification. The time of shifting between points n and p' will be veryshort, owing to the square topped form of the wave.

' As in the case of `Fi .l 3, the filters F' andF may be ein loye to determine the phase relgtions of t e two tubes. Considerable latitude is also possible in the design ico -the curve to the point fn. during one-half05 l iio iis

of the detailedI varrangements for generatl ing the high frequency, for distorting'the'.

high Vfrequency toproduce "al iat toppedv `general prine embodied interconnecting said amplifiers in tandem independently of said line sections, and means to prevent the transmission of audible frequencies over said circuit.

2. In a repeater system, line sections, amplifiers interconnecting said line sections for repeating in opposite directions, means for impressing upon said amplifiers currents to alternately render the amplifiers operative and substantially inoperative, said means comprising a high frequency circuit interconnecting said amplifiers independently of said line sections, and means for pre-venting the transmission of audible frevacuum tube amplifiers interconnecting said line sections for repeating in opposite directions, means to alternately vary the amplication of said amplifiers from an operative to a substantially inoperative condition comrising a circuit arrangement.interconnecting the grids of said amplifiers for the transmission of high frequency controlling currents, and means to prevent the transmission of audible frequencies over said circuit arrangement.

5. In a repeater System, line sections, vacuum tube amplifiers for interconnecting the line sections for repeating in opposite directions, an oscillator, a connection from said oscillator to the grid circuits of each of said amplifiers whereby controlling currents supplied by said oscillator may be alternately a plied to the amplifiers to alternately ren er the amplifiers operative and substantially inoperative, and means in said connection for preventing the transmission of audible frequencies over said connection.

6. In a repeater system, linel sections, vacuum tube amplifiers interconnecting the said line sections for repeating in opposite directions, an oscillator, connections from said oscillator to the grid circuit of each of said amplifiers for applying controlling currents supplied by said oscillator to the grid circuits of the amplifiers to alternately render the amplifiers operative and substantially inoperative, and phase controlling elements in said connections for determining the phase relations with which the controlling currents are applied to the amplifiers.

7. In a repeater system, line sections,

vacuum tube amplifiers interconnecting said line sections, a circuit arrangement interconnecting tlie said amplifiers independently of said line sections for supplying to said amplifiers high frequency currents to alternately render the amplifiers operative and substantially inoperative, and filters in said circuit arrangement for determining the phase relation of the high frequency currents applied to the amplifiers.

' 8. In a repeater system, line sections, vacuum tubeaniplifieis for interconnecting the line sections for repeating in opposite directions, a source of high frequency current, connections from said source to the grid circuits of each vacuum tube amplifier to alternately apply to each tube controlling currents for determining the amplification thereof, and means for determining that the controlling Waves supplied to the amplifiers will be fiat topped.

9. In a repeater system, line sections, vacuum tube amplifiers for interconnecting the line sections for repeating in opposite directions, a source of high frequency current connections'from said source to the grid circuits of each vacuum tube amplifier to alternately apply to each tube controlling currents for determining the amplication thereof, and means for distorting the waves supplied by said source so that the waves will be flat topped. v

10. In a repeater system, line sections, vacuum tube amplifiers interconnecting said line sections for repeating in opposite directions, means to supply square topped waves, circuits interconnecting said means with the grid circuits of the amplifiers, the connection of said circuits with said grid circuits and said means for supplying square topped waves being of such a character as to transmit direct currents, and high pass filters in said circuits having a cut-off point below the fundamental frequency of the square topped wave and above the voice frequencies In testimony whereof, I have signed my name to this specification this 29th day of June 1921.

LLOYD ESPENSCHIEQ. 

